When creating a new catalog, the newly created machines may fail to register with the Cloud delivery controller. We may see the following event in the VDA machines:

Source: Citrix Desktop Service
Event ID: 1001
Level: Error
Description:
The Citrix Desktop Service failed to obtain a list of delivery controllers with which to register. Please ensure that the Active Directory configuration for the farm is correct, that this machine is in the appropriate Active Directory domain and that one or more delivery controllers have been fully initialized. Refer to Citrix Knowledge Base article CTX117248 for further information. Source: Citrix Desktop Service.

C:\Mcspersonality.ini file on the VDAs may show empty value for the ListOfDDCs field:

[StringData]
 $DiskMode=Shared
 [VdaData]
 ListOfDDCs=

In addition, Machine catalog's Provisioning Scheme details may also show that the ControllerAddress field is empty:

Get-ProvScheme -ProvisioningSchemeUid (Get-BrokerCatalog -Name MycatalogName).ProvisioningschemeId

CleanOnBoot                  : False
ControllerAddress            : {}
CpuCount                     : 2
DiskSize                     : 60
HostingUnitName              : DummyNetwork
HostingUnitUid               : e5024667-4ee3-4852-a412-275db0cc2d2f
IdentityPoolName             : DummyCatalogPool
IdentityPoolUid              : a0ea0fdb-7a67-4d07-af23-9887ac1e504d
ImageDefinitionName          :
ImageVersionNumber           :
ImageVersionSpecUid          :
IsVMMetadataEncrypted        : False

HostingUnit associated with the Provisioning Scheme of the machine catalog belonged to a different zone compared to the machine catalog zone. The resource location associated with the HostingUnit Zone did not have any cloud connectors. MCS injects the name of the Cloud connectors present in the same zone as the HostingUnit in to the C:\mcspersonality.ini file. If the Cloud Connectors are not present in the zone, no names will be injected to the mcspersonality.ini file, leading to the registration issue.

Moving the Hosting Unit to the same zone as the Machine Catalog resolved the issue. This can be achieved by right-clicking the Hosting Unit within the Zone, and selecting Move Zone and selecting the target zone.

Note: If there are existing catalogs related to the Hosting Unit, they may also need to be moved to the same target zone. 

VDA machines from a newly created MCS machine Catalog may fail to register with Cloud Delivery Controller because the ControllerAddress field in the ProvisioningScheme is empty. This happens when the environment is configured only for MCS based registration and other options like Registry based, Group Policy based registration are not configured.

Refer to https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/manage-deployment/vda-registration.html#mcs-based for more information on MCS based VDA registration.